The present invention relates to an underwater installation for the production of hydrocarbons on a site the size of which does not allow the building of hydrocarbon treatment and extraction installations and for which it is therefore necessary to arrange for the crude effluent to be drawn off through an underwater pipeline to a central installation which carries out such functions.
Even if they spread over quite a wide area, it is normal practice with underwater hydrocarbon deposits to drain off the major part of the reserves in situ by a central plant which generally consists of one or more platforms or an anchored lighter. The peripheral parts of these deposits, particularly the periclinal areas, are then regarded as being marginal operations; the same applies to small deposits which are often situated close to an operable field.
For these marginal or peripheral parts, as for the small adjacent fields, routing of effluent to a central installation is therefore required. However, if the distance is considerable, several kilometers, if the percentage of gas associated with the oil (in English "Gas Oil Ratio" or GOR) is high, and if it is known that it can fluctuate during the course of production, two major difficulties will become manifest:
the loss of head under diphasic conditions throughout the ducting as a whole may become excessive having regard to the eruptive pressure, PA0 if production under continuous conditions remains possible when this latter is established, restarting of the well or wells after a shut-down may become impossible by reason of the segregation of the liquid phase which results in the formation of a column of degasified liquid which is of excessive hydrostatic weight. PA0 U.S. Pat. No. 3,875,998 (Charpentier) discloses a horizontal separator disposed in the bottom part of a column supported by an articulating arrangement where the difficulties of adjustment linked with the low height of the separator tank were aggravated by the reciprocating nature of the installation. PA0 FR No. 2,026,277 (Ocean Systems) includes horizontal off-shore-converted separators set up directly on the sea bottom, as in the following patent. PA0 FR No. 1,591,780 (Orieux) where the separator is toxic which would be an advantage for connection to the various inlet and outlet pipes, but it does leave the same drawbacks linked with the narrowness of the range of adjustment.
Precautions may be taken against these difficulties by increasing the pressure available at the inlet to the discharge ducting by various means.
An electric pump may be provided at the bottom of the well, taking into account the actual technical limitations inherent in bottom pumps, particularly the limited free gas content at intake, and the risk of frequent maintenance operations (in English "WORK OVER").
It is also possible to set up a gas lift inside the well, a measure which has already been adopted for submerged well operations but which, at a considerable distance from a platform, tends to cause an increase in the head losses in the receiving pipeline and requires a considerable amount of gas to be in circulation by reason of the high average pressure required.
It is also possible to install a diphasic delivery pump near the well-head. This formula has the disadvantage of the low output of diphasic pumps when the GOR increases, which has to be countered by large dimensions and power levels.
As increasing the pressure available at the well-head comes up against serious difficulties as soon as the GOR becomes significant, it has been suggested to avoid these problems by separating the delivery of the gaseous phase from that of the liquid phase by setting up close to the well-head an off-shore production separator. But, such equipment has not been found to be very adaptable to underwater use. In fact, land-based separators are equipment of which the dimensions have been optimised. They require maintaining the level of the oil/gas interface within a narrow range of around 10 of so centimeters and maintaining a high level of separation efficiency calls for frequent adjustments. Finally, with such separators, the height of oil is only around a meter, which hardly facilitates oil pick-up by a conventional type of bottom pump (well bottom pumps, for example).
Thus, many models of off-shore-converted land-based separators have been suggested but have not found practical acceptance, for example: